This invention relates to a method of making oil scraper (oil control) piston rings which have at least one circumferential web portion whose outer circumferential surface lies against and slides along the cylinder wall. To the outer circumferential web surface a wear-resistant layer is applied and is subsequently ground to form sharp edges on the web.
Oil scraper piston rings in a reciprocating piston-type machine serve for distributing an oil film over the cylinder wall. The oil film is just sufficient for lubricating the slide path of the upper piston rings (compression piston rings), and the oil scraper rings deliver the remainder of the oil back into the crankcase. It is an essential requirement that an oil scraper piston ring maintain, independently from the shape of the cylinder, a circumferentially constant surface pressure between the cylinder wall and the runner face of the ring.
Oil scraper piston rings which have an axially narrow circumferential web engaging the cylinder wall are exposed to high surface pressures during operation. For this reason the runner faces of such rings are conventionally provided with a wear-resistant layer, as disclosed, for example, in published European Patent Application 507 636. The piston ring has axially extremely narrow webs so that the surface pressure on the piston ring is very high. Since the wear-resistant layer conventionally constituted by a chromized outer layer of the web does not have the properties required for use in more current engine designs, a hardening of the webs has been effected by nitriding.
Dependent, for example, upon engine type, mode of operation and kind of oil, the oil scraper piston rings may have a service life of 400,000 km in automotive vehicles. Based on the desideratum, particularly in diesel engines, to reach a service life of 1,000,000 km and beyond, wear-resistant layers for compression piston rings have been developed as disclosed, for example, in published European Application 217 126. The wear-resistant layer has, throughout the entire layer volume, firmly embedded ceramic particles which lend the layer an extremely high load bearing capacity and wear resistance. Applying such a layer to conventional oil scraper piston rings has heretofore not been possible because the oil scraper piston rings, after applying a wear-resistant layer to the circumferential web surface, have to be ground in order to obtain a sharp edge which is indispensable for a good oil scraping operation. Since the grinding of the layer has led to the break-off of the ceramic particles, such a layer, although-known by itself, could heretofore not be used for oil scraping piston rings.